Signal integrity with multiple devices.

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
I am attempting to learn more about signal integrity for a
microcontroller design I am working on. I have done simple designs
with just one device attached. Now I want to add more devices. I have
done spice simulations and see the increase in noise and ringing as
more devices are added. What I am not sure about is how to correct it.

Simple series termination would not seem to be sufficient. However
when I look at the few microcontroller designs that have schematics
available, that is all that seems to be used. I have seen nothing that
uses AC termination for example.

I have tried to find information on designing for multiple items
attached to a bus, but it seems to be hard to come by just searching
on google. I find plenty of stuff for one or two devices, but what
about three or more?  Should I use tees or buffers or what. What about
stability when component values are varied.

Any recommendations on sites or books that can help me understand how
to deal with the >3 case would be good.

Re: Signal integrity with multiple devices.
  "db" writes:

Quoted text here. Click to load it

I presume we are talking about a databus system here. If this is for
an RS485 style network then perhaps you should look at the application
notes on Linear Technology's site. They are sometimes quite useful.

If it is any other type of bus come back with more details and you'll
get a number of others responding.

We've slightly trimmed the long signature. Click to see the full one.
Re: Signal integrity with multiple devices.
Quoted text here. Click to load it

Yes this is for a databus. Basically there were between 4-6 devices on
the bus. Specifically the microcontroller(Sharp 79520), sdram,sram ,
flash, fpga,usb, compact flash and maybe a cpld.

I have pulled values from the manufacturers sheet for pin
characteristics and calculated for the impedence of the trace.

So far I have tried series termination and schottky diode termination
in the spice simulation. The results look pretty poor. The terminated
end usually looks pretty good, but the middle would almost certainly
generate double clocking.

I did find a sheet from phillips (AN246) with a pretty good coverage
of different termination issues in a multiport situation and what the
simulations look like. It is good to see some confirmation of some of
the waveforms I saw in my simulations with what is in the Philips app

I guess one specific question I would have is what connection topology
should I be trying to use. So far I have simulated a bus with the
components stubbed in and also in parallel. Should I try some
configuration of tees or a star topology.

Re: Signal integrity with multiple devices.
  "db" writes:


Quoted text here. Click to load it

Quoted text here. Click to load it

So this is a serial device bus similar to IIC.

Essentially keep the traces parallel and ensure they visit each device
in turn (no spurs or T's). You should also attempt to keep each trace
very similar in length. Terminate the end with a suitable termination
network and you should minimise the reflections along the bus.
There are probably app notes with various companies that do these type
of devices. Does the Philips AppNote say anything useful about

We've slightly trimmed the long signature. Click to see the full one.
Re: Signal integrity with multiple devices. ("Paul E. Bennett") wrote in message
Quoted text here. Click to load it

I have been able to find several now that have been helpful and have
been able to get better results from the simulations. One mistake I
was makeing was not accounting for the capacitance of the attached
devices and accounting for how that affects the impedence of the line.

The best notes I found were
AN246 Transmission lines and terminations  with Philips advanced logic

AN1061 Reflecting on transmission line effects.
AN1051 Transmission line effects in pcb applications.

The boards will be quite small. I am shooting for the 3x4 inch size,
so I may not need terminations at all. However I want to be real sure.
I do not want to spend the money to build these boards and then
discover they don't work because of sigint problems.

I still have a more simulation to do, but the problems seem a little
more manageable now.
Based on what I have simulated so far AC termination seems to provide
the best results. Parallel was good also, but I do not want the power
drain. I saw a lot of ringing with diode termination. Series seemed to
be the worst, but that does not seem to be recommended except in point
to point situations.

Site Timeline